1. Field of the Invention
The present invention relates to a piezoelectric bulk wave filter which can be used as a bandpass filter, for example. More specifically, the present invention relates to a longitudinally coupled multi-mode piezoelectric bulk wave filter device, a longitudinally coupled multi-mode piezoelectric bulk wave filter, and an electronic component including such a piezoelectric bulk wave filter using a harmonic wave of vibration. The present invention also relates to a longitudinally coupled multi-mode piezoelectric bulk wave filter device and an electronic component including a piezoelectric bulk wave filter which couples harmonic waves of different orders.
2. Description of the Related Art
A variety of piezoelectric bulk wave filters are used as bandpass filters. Dual-mode piezoelectric bulk wave filters are mainly used within a frequency range of several MHz to tens of MHz because of the ease in which they can be miniaturized and their low cost.
A dual-mode piezoelectric bulk wave filter is disclosed in Japanese Unexamined Patent Application Publication No. 5-327401.
FIG. 18 is a cross-sectional view of a conventional dual-mode piezoelectric bulk wave filter using a thickness longitudinal vibration.
A piezoelectric bulk wave filter 201 includes a piezoelectric substrate 202 that is polarized in the thickness direction. A pair of exciting electrodes 203 and 204 are disposed on the top surface of the piezoelectric substrate 202. A common exciting electrode 205 is opposed to the exciting electrodes 203 and 204 with the piezoelectric substrate 202 sandwiched therebetween.
During use, an input signal is applied between one exciting electrode 203 and the common exciting electrode 205 to excite the piezoelectric substrate 202. When the piezoelectric substrate 202 is excited, a symmetrical mode shown in FIG. 19A and an anti-symmetrical mode shown in FIG. 19B are generated. The two modes are coupled, forming a filter bandwidth. An output is picked up between the exciting electrode 204 and the ground electrode 205.
Also known, in addition to the dual-mode piezoelectric bulk wave filter operating in the thickness longitudinal mode, is the dual-mode piezoelectric bulk wave filter which includes the piezoelectric substrate 202 polarized in a direction parallel to the top surface and operating in a shear vibration mode.
The degree of coupling between the symmetrical mode and anti-symmetrical mode in the conventional piezoelectric bulk wave filter 201 depends on the spacing between the exciting electrodes 203 and 204. The spacing determines a frequency difference between the symmetrical mode and the anti-symmetrical mode, thereby determining a passband.
Specifically, to produce a wide band filter, the spacing between the exciting electrodes 203 and 204 must be narrowed to increase the degree of coupling between the two modes and to increase the frequency between the two modes.
The exciting electrodes 203 and 204 are typically produced using a screen printing of electrically conductive paste. The screen printing technique has limited ability to narrow the spacing between the exciting electrodes 203 and 204. If the exciting electrodes 203 and 204 are produced using a photolithographic technique, the spacing between the exciting electrodes 203 and 204 is narrowed, but the costs involved increase.
Even if the spacing between the exciting electrodes 203 and 204 is narrowed, the capacitance between the exciting electrodes 203 and 204 increases in the piezoelectric bulk wave filter 201, which leads to a smaller attenuation.
To attain a large attenuation, a plurality of filter elements are typically connected in the piezoelectric bulk wave filter device. As shown in FIG. 20, first and second piezoelectric bulk wave filter elements 213 and 214 are mounted on a substrate 212 in a piezoelectric bulk wave filter device 211. The piezoelectric bulk wave filter element 213 and piezoelectric bulk wave filter element 214 are identical to each other in construction.
If the first and second piezoelectric bulk wave filter elements 213 and 214 are located too closely, a stray capacitance occurring between input and output of the first and second piezoelectric bulk wave filter elements 213 and 214 (as represented by an arrow A shown in FIG. 20) lowers the attenuation. For this reason, the first and second piezoelectric bulk wave filter elements 213 and 214 are not located too closely to each other. The whole filter device inevitably becomes large in size.
As another method to achieve a large attenuation, a piezoelectric bulk wave filter which includes a relay capacitor as shown in FIG. 21 has been proposed. As shown, a piezoelectric bulk wave filter 221 includes first and second energy trapped piezoelectric resonators 222 and 223 disposed on a piezoelectric substrate. To form a relay capacitor between the piezoelectric resonators 222 and 223, capacitor electrodes 224 and 225 are opposed to each other with the piezoelectric substrate sandwiched therebetween. FIG. 22 is a circuit diagram of the piezoelectric bulk wave filter 221 shown in FIG. 21.
The relay capacitor C is formed of the capacitor electrodes 224 and 225 as shown in FIG. 22. Because the piezoelectric substrate forming the piezoelectric bulk wave filter 221 is polarized, an unwanted vibration occurs because of the piezoelectric effect on the portions of the piezoelectric substrate where the capacitor electrodes 224 and 225 are disposed. As a result, spurious vibrations occur.
A technique is known to partially polarize the piezoelectric substrate so that the piezoelectric substrate is not polarized in the portions where the capacitor electrodes 224 and 225 are opposed. However, if the piezoelectric substrate is partially polarized, there is a possibility that cracks may occur in the piezoelectric substrate.
In order to overcome the problems described above, preferred embodiments of the present invention provide a multi-mode piezoelectric bulk wave filter device, a multi-mode piezoelectric bulk wave filter, and an electronic component including the multi-mode piezoelectric bulk wave filter which has a wide bandwidth, has a large attenuation, is easy to miniaturize, and suppresses the generation of unwanted spurious vibrations. Preferred embodiments of the present invention also provide a longitudinally coupled multi-mode piezoelectric bulk wave filter that is manufactured at low costs.
In a first preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric bulk wave filter device includes first and second multi-mode piezoelectric bulk wave filters which are designed so that vibration modes of different orders of harmonic waves are excited and coupled to provide an output signal between an output electrode and an ground electrode when an input signal is input between an input electrode and the ground electrode. The longitudinally coupled multi-mode piezoelectric bulk wave filter device further includes a casing substrate on which the first and second longitudinally coupled multi-mode piezoelectric bulk wave filters are disposed on a surface thereof. Each of the first and second multi-mode piezoelectric bulk wave filters includes at least four exciting electrodes extending substantially parallel to each other and a laminated piezoelectric body, including a plurality of piezoelectric layers arranged between the exciting electrodes, and is polarized in a direction that is substantially perpendicular to or substantially parallel to the exciting electrodes.
The laminated piezoelectric body has first and second end surfaces facing in a direction that is substantially perpendicular to the plurality of the piezoelectric layers and first through fourth side surfaces connecting the first and second end surfaces. The input electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body and closer to the first end surface. The output electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body and closer to the second end surface. The ground electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body.
A plurality of exciting electrodes includes a first group which is connected to the input electrode, a second group which is connected to the output electrode, and a third group which is connected to the ground electrode. The first and second multi-mode piezoelectric bulk wave filters are cascaded and arranged so that the distance between the ground electrodes of the first and second multi-mode piezoelectric bulk wave filters is smaller than the distance between the input electrodes of the first and second multi-mode piezoelectric bulk wave filters and the distance between the output electrodes of the first and second multi-mode piezoelectric bulk wave filters to each other.
The multi-mode piezoelectric bulk wave filter according to the first preferred embodiment is not limited to a particular vibration mode. The multi-mode piezoelectric bulk wave filter may use a length vibration node or a thickness vibration mode.
Preferably, the longitudinally coupled multi-mode piezoelectric bulk wave filter device further includes a reflective layer, which is connected to the first and second end surfaces of the laminated piezoelectric body and is preferably fabricated of a material having an acoustic impedance Z2 lower than an acoustic impedance Z1 of the piezoelectric material of the laminated piezoelectric body.
Support members are connected to the surfaces of the reflective layers opposite from the surfaces thereof connected to the first and second end surfaces of the laminated piezoelectric body and are preferably fabricated of a material having a acoustic impedance Z3 higher than the acoustic impedance Z2 of the reflective layers.
In this arrangement, a vibration transferred to the reflective layer from the laminated piezoelectric body is reflected from the boundary between the reflective layer and the support member. The support member thus mechanically supports the laminated piezoelectric body without affecting the vibration characteristics of the laminated piezoelectric body. The vibration mode of the laminated piezoelectric body is thus subject to less constraints.
In a second preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric bulk wave filter includes at least four exciting electrodes extending substantially parallel to each other and a laminated piezoelectric body, including a plurality of piezoelectric layers arranged between the exciting electrodes, and is polarized in a direction that is substantially perpendicular to or substantially parallel to the exciting electrodes. The laminated piezoelectric body has first and second end surfaces facing in a direction that is substantially perpendicular to the plurality of the piezoelectric layers and first through fourth side surfaces connecting the first and second end surfaces.
An input electrode is disposed on the first side surface of the laminated piezoelectric body closer to the first end surface. An output electrode is disposed on the first side surface of the laminated piezoelectric body closer to the second end surface. A ground electrode is disposed on the second side surface of the laminated piezoelectric body opposite from the first side surface.
A plurality of exciting electrodes includes a first group which is connected to the input electrode, a second group which is connected to the output electrode, and a third group which is connected to the ground electrode.
Vibration modes of different orders of harmonic waves are excited and coupled in response to an input signal input between the input electrode and the ground electrode so that an output signal is output between the output terminal and the ground terminal.
First and second reflective layers are respectively arranged on the first and second end surfaces of the laminated piezoelectric body and have an acoustic impedance Z2 lower than an acoustic impedance Z1 of the laminated piezoelectric body.
First and second support members are respectively arranged on the first and second reflective layers and have an acoustic impedance Z3 higher than the acoustic impedance Z2 of the reflective layers.
A capacitor unit is located on the first support member and/or the second support member so that the capacitor unit is connected between the ground electrode and one of the output electrode and the input electrode.
The capacitor unit may be constructed in a variety of forms. Preferably, the capacitor unit includes a pair of capacitor electrodes respectively disposed on a pair of external opposed surfaces of the support member.
Preferably, the capacitor unit includes a plurality of internal electrodes being laminated with each other with the support member layers interposed therebetween.
The multi-mode piezoelectric bulk wave filter according to the second preferred embodiment is not limited to a particular vibration mode. The multi-mode piezoelectric bulk wave filter may use a length vibration mode or a thickness vibration mode.
In a third preferred embodiment of the present invention, an electronic component includes a casing substrate, the longitudinally coupled multi-mode piezoelectric bulk wave filter according to the second preferred embodiment mounted on the casing substrate, and a cap member secured to the casing substrate such that the cap surrounds the piezoelectric bulk wave filter.
In a fourth preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric bulk wave filter device includes first and second multi-mode piezoelectric bulk wave filters which are designed so that vibration modes of different orders of harmonic waves are excited and coupled to provide an output signal between an output electrode and a ground electrode when an input signal is input between an input electrode and the ground electrode. Each of the first and second multi-mode piezoelectric bulk wave filters includes at least four exciting electrodes extending substantially parallel to each other and a laminated piezoelectric body, including a plurality of piezoelectric layers arranged between the exciting electrodes and polarized in a direction substantially perpendicular to or substantially parallel to the exciting electrodes.
The laminated piezoelectric body has first and second end surfaces facing in a direction that is substantially perpendicular to the plurality of the piezoelectric layers and first through fourth side surfaces connecting the first and second end surfaces.
The input electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body closer to the first end surface. The output electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body closer to the second end surface. The ground electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body.
The plurality of exciting electrodes includes a first group which is connected to the input electrode, a second group which is connected to the output electrode, and a third group which is connected to the ground electrode.
The first and second multi-mode piezoelectric bulk wave filters are cascaded, and the order of the harmonic waves of the mode used by the first longitudinally coupled multi-mode piezoelectric bulk wave filter is different from the order of the harmonic waves of the mode used by the second longitudinally coupled multi-mode piezoelectric bulk wave filter.
In a fifth preferred embodiment of the present invention, a longitudinally coupled multi-mode piezoelectric bulk wave filter device includes first and second multi-mode piezoelectric bulk wave filters which are designed so that vibration modes of different orders of harmonic waves are excited and coupled to provide an output signal between an output electrode and a ground electrode when an input signal is input between an input electrode and the ground electrode.
Each of the first and second multi-mode piezoelectric bulk wave filters includes at least four exciting electrodes extending substantially parallel to each other and a laminated piezoelectric body, including a plurality of piezoelectric layers arranged between the exciting electrodes and polarized in a direction substantially perpendicular to or substantially parallel to the exciting electrodes.
The laminated piezoelectric body has first and second end surfaces facing in a direction that is substantially perpendicular to the plurality of the piezoelectric layers and first through fourth side surfaces connecting the first and second end surfaces.
The input electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body closer to the first end surface. The output electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body closer to the second end surface. The ground electrode is disposed on at least one of the first through fourth side surfaces of the laminated piezoelectric body.
The plurality of exciting electrodes includes a first group which is connected to the input electrode, a second group which is connected to the output electrode, and a third group which is connected to the ground electrode. The first and second multi-mode piezoelectric bulk wave filters are cascaded, and the first longitudinally coupled multi-mode piezoelectric bulk wave filter and the second longitudinally coupled multi-mode piezoelectric bulk wave filter have different dimensions.
The first longitudinally coupled multi-mode piezoelectric bulk wave filter and the second longitudinally coupled multi-mode piezoelectric bulk wave filter may be different in width dimensions, in length dimensions, in both width or length, or in any other dimensions.
The longitudinally coupled multi-mode piezoelectric bulk wave filter device may further include a third longitudinally coupled multi-mode piezoelectric bulk wave filter cascaded with the first and second longitudinally coupled multi-mode piezoelectric bulk wave filters. The filter device may include three or more cascaded longitudinally coupled multi-mode piezoelectric bulk wave filters.
The multi-mode piezoelectric bulk wave filter device according to the fifth preferred embodiment is not limited to a particular vibration mode. The multi-mode piezoelectric bulk wave filter may use a harmonic wave of a length or thickness vibration mode.
Preferably, the longitudinally coupled multi-mode piezoelectric bulk wave filter device further includes reflective layers connected to the first and second end surfaces of the laminated piezoelectric body and preferably fabricated of a material having a acoustic impedance Z2 lower than an acoustic impedance Z1 of a piezoelectric material of the laminated piezoelectric body.
Support members are connected to the surfaces of the reflective layers opposite from the surfaces thereof connected to the first and second end surfaces of the laminated piezoelectric body and are preferably fabricated of a material having a acoustic impedance Z3 higher than the acoustic impedance Z2 of the reflective layers. In this arrangement, a vibration transferred to the reflective layer from the laminated piezoelectric body is reflected from the boundary between the reflective layer and the support member. Because a vibration energy is trapped in a region of the boundary between the reflective layer and the support member, the support member mechanically supports the piezoelectric bulk wave filter without adversely affecting the filter characteristics.
Preferably, a groove is formed on at least one side surface of the laminated piezoelectric body of at least one of the first and second multi-mode piezoelectric bulk wave filters, and a pair of multi-mode piezoelectric bulk wave filter blocks are integrally formed on both sides of the groove. In this arrangement, at least one of the first and second multi-mode piezoelectric bulk wave filters is a two-element piezoelectric bulk wave filters including a pair of longitudinally coupled multi-mode piezoelectric bulk wave filter blocks which provides a large attenuation.
In a sixth preferred embodiment of the present invention, an electronic component includes the longitudinally coupled multi-mode piezoelectric bulk wave filter device, a casing substrate bearing the longitudinally coupled multi-mode piezoelectric bulk wave filter device, and a cap member secured to the casing substrate in such a manner that the cap surrounds the longitudinally coupled multi-mode piezoelectric bulk wave filter device.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.